With a clear horizon and the sun setting over the sea, it happens regularly.
Maybe heat waves of a too warm sea blur it too much. I don’t know as where I am, the sun sets over the coast of UK in summer, so no green flash possible. This was probably my last sighting before October.
If the weather in the Opal Coast was a bit more predictable, I could set up a Green Flash NG meeting but alas, there is no reliable weather pattern there in Spring and Fall.